AMIDES OF ACIDS OF PHOSPHORUS FOR THE EXTRACTION OF METALS

Abstract: Compounds of formula (I), wherein R<1> is an optionally substituted 2-alkylphenoxy group, each of R<2>, R<3> and R<4> is a group selected from optionally substituted 2-alkylphenoxy, optionally substituted phenyl, optionally substituted alkyl and optionally substituted alkoxy and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R<1> and R<2> is 2-isopropyl-4-<u>tert</u>-nonylphenoxy and each of R<3> and R<4> is phenyl and the compound wherein R<1> is 2-methyl-4-<u>tert</u>-nonylphenoxy, R<2> is 2,4-dimethylphenoxy and each of R<3> and R<4> is phenyl. Compounds of formula (I) are useful for the solvent extraction of metals, especially zinc from aqueous solution.

Description: AMIDES OF ACIDS OF PHOSPHORUS FOR THE EXTRACTION OF METALS This invention relates to chemical compounds and more particularly to certain aryloxy-substituted amidobis (thiophosphoryl) compounds and to their use as extractants in a solvent extraction process for extracting metal values from aqueous solutions of metal salts. The use of solvent extraction techniques for the hydrometallurgical recovery of metal values from metal ores has been practised commercially for a number of years. In general, the technique involves contacting an aqueous solution of metal salt, obtained for example by treating the crushed ore with acid, with a solution in a water-immiscible organic solvent of an organic extractant which complexes with the metal and extracts it into the non-aqueous phase. The metal may then be recovered by a further extraction step in which the organic solution containing the metal complex is contacted with another aqueous phase containing an agent, usually a strong acid, capable of decomposing the complex so that the metal is extracted into the aqueous phase from which it can be recovered by suitable procedures such as electrowinning. Since metals are usually found in their ores in association with other metals, it is essential that the organic extractant extracts the desired metal selectively so as to achieve a degree of separation from the other metals present. Selective extractants are known for some metals, for example copper, and their use is well-established. The search for a suitably selective extractant for zinc has been less successful. The use of extractants containing the phosphoric acid group, especially di (2-ethylhexyl)phosphoric acid (D2EPHA) , has been proposed, see "Productivity and Technology in the Metallurgical Industries", edited by M.Koch and J.C.Taylor, in an article by A.Selke and D.de Juan Garcia, pages 695 to 703. However, as is apparent from the Selke et al article, ferric iron is extracted together with the zinc. To prevent build-up of the ferric iron in the organic solution, it is necessary to remove the iron from the organic solution in a stripping stage subsequent to that used to recover the zinc. In this separate stripping stage, the organic solution is contacted with 5 to 6 molar hydrochloric acid to give ferric chloride in hydrochloric acid. Free hydrochloric acid is recovered by a further step in which the ferric chloride in hydrochloric acid is contacted with an organic solution containing tributyl phosphate from which the ferric chloride is stripped using water. The additional stages required to remove iron add to the complexity and cost of the procedure and hence are undesirable. The behaviour of bis (2,4,4-trimethylpentyl) onothiophosphinic acid as an extractant for zinc has been studied by C.Caravaca and F.J.Alguacil (Hydrometallurgy, .27, 1991, 327-338) who found that at relatively acidic pH values (near 1.0) the proportion of zinc extracted was only slightly higher than the proportion of iron. It has now been found that certain compounds containing the aryloxy-substituted amidobis (thiophosphoryl) group are excellent metal extractants, particularly for the separation of other metals from solutions containing iron. In particular, it has been found that these amidobis (thiophosphoryl) compounds are highly effective in selectively extracting zinc from acidic aqueous solutions containing both zinc (II) and iron(III) ions. It has also been found that certain of the amido-bis (thiophosphoryl) compounds are strong extractants, that is that they extract zinc from aqueous solutions at pH values below 2 without requiring the addition of base to neutralise the protons liberated by the complexation reaction. Accordingly, the present invention provides a compound of the formula: R 1 ^ S S / R 3 P NH P R 2 R wherein R 1 is an optionally substituted 2-alkylphenoxy group, each of R 2 , R 3 and R 4 is a group selected from optionally substituted 2- alkylphenoxy, optionally substituted phenyl, optionally substituted alkyl and optionally substituted alkoxy and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R 1 and R 2 is 2-isopropyl-4-tert- nonylphenoxy and each of R 3 and R 4 is phenyl and the compound wherein R 1 is 2-methyl-4-tert-nonylphenoxy, R 2 is 2,4-dimethylphenoxy and each of R 3 and R 4 is phenyl. It will be appreciated that the structure of the compounds of Formula I is such that they may exist in more than one tautomeric form, another such form having the structure: wherein R 1 -R 4 are as already defined. Whilst the invention is described herein with reference to compounds of Formula I, it is to be understood that it relates to said compounds in any of their possible tautomeric forms . As examples of optionally substituted 2-alkylphenoxy groups which may be present in the compounds of Formula I, there may be mentioned phenoxy groups wherein the alkyl substituent in the 2- position relative to the oxygen atom contains from 1 to 20, for example from 1 to 10, carbon atoms. Such alkyl groups may be primary alkyl groups having one or more carbon atoms, secondary alkyl groups having three or more carbon atoms or tertiary alkyl groups having four or more carbon atoms. In addition to the alkyl substituent in the 2- position, the phenoxy residue may optionally carry one or more additional alkyl substituents, for example an alkyl substituent in the 4-position. When the optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent, the latter may be present in addition to the alkyl substituent in the 2-position and/or the alkyl substituent in the 2-position may itself be a tertiary alkyl group. Examples of optionally substituted 2-alkylphenoxy groups include 2-1ert-butvlphenoxv. 2-tert-butvl- -methvlphenoxv. 2-tert- butyl-5-methylphenoxy, 2,4-di-tert-butvlphenoxv, 2,4-di-tert- pentylphenoxy, 2-methyl-4-tert-nonylphenoxy, 2-tert-butyl-4-tert- nonylphenoxy, 4-octylphenoxy, 4-tert-dodecylphenoxv, 4-tert-dodecyl-2- methylphenoxy, 2-sec-butylphenoxy and the like. In particularly useful compounds of Formula I, at least one 2-alkylphenoxy group is a 2-tert-alkylphenoxy group and preferably at least two 2-alkylphenoxy groups are 2-tert-alkylphenoxy groups. Preferred 2-tert-alkvl groups include 2-tert-butyl groups. Optionally substituted phenyl groups which may be represented by R 2 and/or R 3 and/or R 4 in the compounds of Formula I include alkyl substituted phenyl groups, for example o-tolyl, -tolyl, p-tolyl and xylyl groups and mixtures of such groups. However, because of the commercial availability of suitable intermediates, the preferred optionally substituted phenyl group is the unsubstituted phenyl group. As examples of optionally substituted alkyl and optionally substituted alkoxy groups which may be represented by R 2 and/or R 3 and/or R 4 , there may be mentioned alkyl and alkoxy groups containing from 1 to 20, for example from l to 10, carbon atoms. The alkyl groups and the alkyl moieties of alkoxy groups may be primary alkyls having one or more carbon atoms, secondary alkyls containing three or more carbon atoms or tertiary alkyls containing four or more carbon atoms. As examples of substituents which may be present in substituted alkyl or substituted alkoxy groups, there may be mentioned halogen, nitro, cyano, hydrocarbyloxy, hydrocarbyloxycarbonyl, acyl and acyloxy groups. More than one substituent may be present in which case the substituents may be the same or different. One valuable class of compounds falling within the scope of Formula I comprises the compounds wherein R 1 is an optionally substituted 2-alkylphenoxy group, each of R 2 , R 3 and R 4 is either an optionally substituted 2-alkylphenoxy group or an optionally substituted phenyl group and at least one optionally substituted 2- alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R 1 and R 2 is 2-isopropyl-4-tert- onylphenoxy and each of R 3 and R 4 is phenyl and the compound wherein R 1 is 2- methyl-4-.tert-nonylphenoxy, R 2 is 2,4-dimethylphenoxy and each of R 3 and R 4 is phenyl. When only R 1 in this class of compounds is an optionally substituted 2-alkylphenoxy group, R 2 , R 3 and R 4 being phenyl groups, it is preferred that the phenoxy group is heavily substituted with aliphatic groups as in, for example, the 2-tert-butyl-4-tert- nonylphenoxy group to provide the extractant compound with good solubility in hydrocarbon solvents. It is preferred, in this class of compounds, that at least one of R 2 , R 3 and R 4 is an optionally substituted 2-alkylphenoxy group so that the compound of Formula I contains at least two optionally substituted 2-alkylphenoxy groups and it is further preferred that at least two optionally substituted 2-alkylphenoxy groups have a tertiary alkyl substituent. Compounds of Formula I in which R 1 and R 2 are optionally substituted 2-alkylphenoxy groups, R 3 and R 4 being optionally substituted phenyl are strong metal extractants. In an example of such a compound, each of R 1 and R 2 is 2-methyl-4-tert-nonylphenoxy and each of R 3 and R 4 is phenyl. Useful compounds in which R 1 and R 3 are optionally substituted 2-alkylphenoxy groups, R 2 and R 4 being optionally substituted phenyl, include compounds in which the alkyl substituents in the 2-position are tertiary alkyl groups, for example the compound in which each of R 1 and R 3 is 2-tert-butyl-4-methvlphenoxy and each of R 2 and R 4 is phenyl. In especially valuable compounds of Formula I, each of R 1 , R 2 and R 3 is an optionally substituted 2-alkylphenoxy group and R 4 is optionally substituted phenyl. Preferably, at least one of the 2- alkylphenoxy groups is a 2-ter -alkylphenoxy group, the others preferably being 2-tert-alkyl and/or 2-sec-alkylphenoxy. preferred sec-alkyl groups having at least four carbon atoms. Examples of compounds containing one 2-tert-alkyl substituent and two 2-sec-alkyl substituents include the compound wherein each of R 1 and R 2 is 2-sec- butylphenoxy, R 3 is 2,4-di-tert- entvlphenoxv and R 4 is phenyl. Examples of compounds containing two 2-tert-alkyl substituents and one 2-sec-alkyl substituent include the compound wherein each of R 1 and R 3 is 2-tert-butvlphenoxv, R 2 is 2-sec-butvlphenoxv and R 4 is phenyl. In compounds of Formula I in which each of R 1 , R 2 , R 3 and R 4 is an optionally substituted 2-alkylphenoxy group, preferably at least one is a 2-tert-alkylphenoxy group and more preferably two and especially three of the optionally substituted 2-alkylphenoxy groups are 2-tert-alkylphenoxy groups. Useful structures include those compounds in which R 1 and R 2 are 2-tert-alkylphenoxy groups especially when R 3 and R 4 are 2-sec-alkylphenoxy groups. A second valuable class of compounds falling within the scope of Formula I comprises the compounds wherein R 1 is an optionally substituted 2-alkylphenoxy group, at least one of R 2 , R 3 and R 4 is an optionally substituted alkyl or optionally substituted alkoxy group, any remaining group or groups from R 2 , R 3 and R 4 being selected from optionally substituted 2-alkylphenoxy and optionally substituted phenyl and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent. Within this second class of compounds, mention may be made of compounds of Formula I in which R 1 is an optionally substituted 2- tert-alkylphenoxy group, R 2 is an optionally substituted alkoxy group and each of R 3 and R 4 , independently, is an optionally substituted 2- alkylphenoxy group or an optionally substituted alkoxy group. Preferably, at least one of R 3 and R 4 is a 2-tert-alkylphenoxy group. Also within this second class of compounds, mention may be made of compounds of Formula I in which R 1 is an optionally substituted 2-tert-alkylphenoxy group, R 2 is an optionally substituted alkoxy group or an optionally substituted phenyl group and R 3 and R 4 are optionally substituted alkoxy groups which may be the same or different. Also within this second class of compounds, mention may be made of compounds of Formula I in which R 1 is an optionally substituted 2-tert-alkylphenoxy group, R 2 is an optionally substituted 2-alkylphenoxy group or an optionally substituted phenyl group and R 3 and R 4 are optionally substituted alkyl groups which may be the same or different. Compounds of Formula I may be obtained from a chlorophosphorus compound of formula A, B, C or D \ \ R \l -cι -c, • cι / / / A chloro compound of formula C or D may be reacted with an excess of ammonia to give a phosphoramide, E. The amide E may be reacted with a chloro compound C or D 10 in the presence of a strong base. —* ■ fi ' \ .f?l N ■ — l Pl/" ! + H ? * N a C I Instructions for the preparation of amides (cf E) and 20 reaction with chloro compounds for oxygenated analogues are given by L.Meznik and A.Maracek, Z.Chem. 21(8), 1981 at page 259, but more satisfactory reaction conditions are provided herein. General methods for preparation of the chloro compounds C- D are well known to the art, for example when R 1 -R 2 are aryl groups, 25 Grignard reagents may be reacted with phosphorus trichloride (if required in two stages, to give a mixed product) as described by W.Voskuil and J.F.Arens, Rec.Trav.Chim. , 82., 302, (1963): -_ Q , , R 2 MgCI \ ϋ R 1 MgCI +PC I 3 -→-R 1 PC I 2 * ■ P Cl / and the diarylchlorophosphine may be oxidised by reaction with 35 thiophosphoryl chloride to give C. Alternatively, 3 equivalents of a Grignard reagent may be reacted with a dialkyl phosphite to give a dialkyl phosphine oxide which is converted into the acid chloride by reaction with phosphorus trichloride as described by Robert H Williams and Lyle A Hamilton, 40 J.Am.Chem.Soc. , 74., 5418, 1952. : 4 H 9°\ \ PC 3R 1 gB PH PH / / C,,HgO R 1 P — c Useful available compounds of formula A or B include chlorodiphenylphosphine. To prepare compounds of formulae C and D in which R 1 and R 2 are aryloxy, a dithiophosphoric acid may be reacted with chlorine or sulphuryl chloride: as described by John H.Fletcher et al, J. m.Chem.Soc. , 72., 2461, 1950. Where R^R 4 are aryloxy groups, a wide range of dithiophosphoric acids for use in the reaction above may be prepared by reaction of the appropriate phenol either with phosphorus pentasulphide or with thiophosphoryl chloride in the presence of an acid acceptor. When thiophosphoryl chloride is used, different phenols may be reacted sequentially to provide compounds C and D in which R 1 is different from R 2 and R 3 is different from R 4 . 4 R 1 H + P 2 S 5 \fi. / ■ SH + H 2 5 R 1 Fletcher, loc cit) See N.A.Meinhardt, S.Z.Cardon and P.W.Vogel, J.Org.Chem. , 25, 1991, (1960) and J.H.Fletcher et al, J.Am.Chem. Soc, 70., 3943, (1948) . Compounds of formula C in which R 1 is phenyl and R 2 is aryloxy may be obtained by reacting C 6 H S PSC1 2 with a phenol. In a further aspect, the present invention provides a process for extracting metal values from aqueous solutions of metal salts with an organic phase comprising a compound of the invention as hereinbefore defined. The organic phase employed in the extraction process typically contains a water-immiscible inert organic solvent, that is to say a water-immiscible organic liquid that is inert under the extraction conditions and is a good solvent for the extractant compound and the metal complex thereof. It will be appreciated that the extraction process may be incorporated into a wide variety of different procedures for the recovery of metals from their ores or from other metal-bearing sources. Details of these procedures will vary depending on the metal concerned and the nature and composition of the leach solution. An integrated process which is especially suitable for sulphate leach solutions can be carried out using operations well known to the skilled person. Typically, the extractive process comprises a sequence of stages in which the metal is extracted into an organic solution, stripped into an aqueous phase and recovered from the aqueous phase by any suitable means, for example by electrowinning. Thus, as a particular aspect of the invention, there is provided a process for extracting metal values from aqueous solution by a sequence of stages comprising: (1) contacting the aqueous solution containing metal values with a solution of an extractant compound as hereinbefore defined in a water-immiscible organic solvent whereby to extract metal values into the solvent in the form of a complex of the metal with the extractant; (2) separating the solvent phase containing metal complex from the extracted aqueous phase,* (3) contacting the solvent phase containing metal complex with an aqueous strip solution whereby the metal complex is unstable and metal ions transfer into the aqueous phase, and (4) separating the aqueous phase containing metal ions from the stripped solvent phase. The extraction process may be applied to the extraction from aqueous solution of any metal capable of forming a stable complex with a compound of the invention in the organic phase. The process is especially suitable for the solvent extraction of zinc from aqueous solutions of zinc salts, especially solutions obtained by the acid leaching of zinc ores. Examples, however, of other metals which can be extracted from acidic solutions having pH values of pH 2 and below are bismuth, cadmium, silver, mercury and copper but many other metals may also be extracted at higher pH values. In operating stage (1) of the aforementioned process, the amount of extractant compound to be used will depend upon the concentration of metal salt in the aqueous solution and also on the plant design. It is preferred, however, to use from 5g to 400g of compound of the invention per dm 3 (litre) of organic solution. Higher concentrations may be used but tend to afford organic phases of too high viscosity for convenient handling. Lower concentrations can also be used but involve the use of unnecessarily large volumes of solvent. For use with aqueous solutions containing lg or more per dm 3 of a metal such as zinc, it is preferred to use from 50 to 400g of compounds of the invention per dm 3 of organic solution. If desired, the extractant compound can be used together with an agent which modifies the behaviour thereof in the extraction process, for example an alkylphenol, alcohol or ester which may be used in an amount of from 10% to 200%, especially from 20% to 100% by weight of extractant compound. Such compounds weaken the extractant but facilitate the subsequent stripping of metal therefrom. In this way, a very strong extractant may be adjusted in strength to the requirements of different feed solutions and different stripping solutions. Alkylphenols which may be used as modifiers in conjunction with the extractant compounds of the invention include alkylphenols containing from 3 to 15 alkyl carbon atoms, for example 4-tert- butylphenol, 4-heptylphenol, 5-methyl-4-pentylphenol, 2-chloro-4- nonylphenol, 2-cyano-4-nonylphenol, 4-dodecylphenol, 3- pentadecylphenol and 4-nonylphenol and mixtures thereof. The preferred phenols contain alkyl groups having from 4 to 12 carbon atoms, especially the mixed 4-nonylphenols obtained by condensation of phenol and propylene trimer. Alcohols which may be used as modifiers in conjunction with the extractant compounds of the invention include saturated and unsaturated hydrocarbon alcohols and polyols containing 14 to 30, preferably 15 to 25 carbon atoms. The alcohols are preferably highly branched with the hydroxyl group located approximately midway along the hydrocarbon backbone. Especially preferred are the branched chain alcohols that may be made by condensation of short chain alcohols by the Guerbet process, such alcohols sometimes being referred to as Guerbet alcohols. Optionally, the alcohols may contain an aromatic group or other functional group, particularly an ester group. Especially useful alcohols may be synthesised from highly branched precursors leading to very highly branched Guerbet alcohols containing a large number of terminal methyl groups. Examples of particularly efficient alcohol modifiers include highly branched isohexadecyl alcohol and iso-octadecyl alcohol, the latter being 2- (1,3,3-trimethylbutyl) -5,7,7-trimethyloctanol. Esters which may be used as modifiers in conjunction with the extractant compounds of the invention include saturated and unsaturated aliphatic and aromatic-aliphatic esters containing from 10 to 30 carbon atoms. The esters may be mono-esters or polyesters, especially di-esters. The esters are preferably highly branched. Optionally, the esters may contain other functional groups, particularly a hydroxyl group. Especially useful esters include 2,2,4-trimethyl-l,3-pentanediol isobutyrate and the benzoic acid ester thereof. In the context of the present invention, 'highly branched' as applied to the alcohols and esters means that the ratio of the number of methyl carbon atoms to non-methyl carbon atoms is higher than 1:5. Preferably, this ratio is higher than 1:3. If desired, mixtures of alkylphenols and/or alcohols and/or esters may be employed as modifiers. The aforementioned modifiers may be used in the preparation of extractant compositions containing one or more extractant compound of the invention and one or more modifier. It has been found that for some of the pure extractant compounds of the invention the rate at which zinc is extracted is rather slow, but that a wide range of compounds may be added to increase this rate even in amounts of 1.0% and below. Useful rate increasing additives include compounds which are known to be extractants for zinc and are soluble in the organic phase but with fast rates of extraction. Compounds found to be effective in this aspect are other known zinc extractants such as esters of phosphoric acid, (eg D2EHPA) , and particularly surface active agents capable of transferring metal ions such as alkyl and aryl sulphonic acids having solubility in the organic phase. Stages (1) and (2) of the aforementioned process may conveniently be carried out using well known conventional solvent extraction techniques. Typically, the aqueous solution containing metal values is intimately contacted, in a single stage or in multiple stages but preferably continuously, with the organic phase (for example by agitating the two phases together in a suitable vessel) for a time sufficient to allow substantial extraction of the metal values from the aqueous solution, the two phases then being separated in any conventional manner. The extraction is usually carried out at ambient temperature although somewhat higher temperatures, for example up to 100°C but preferably not more than 50°C, may be used if operationally convenient. Organic solvents which may be used in the extraction include any mobile organic solvent, or mixture of solvents, which is immiscible with water and is inert under the extraction conditions to the other materials present. Examples of suitable solvents include aliphatic, alicyclic and aromatic hydrocarbons and mixtures of any of these as well as chlorinated hydrocarbons such as trichloroethylene, perchloroethylene, trichloroethane and chloroform. Preferred solvents are hydrocarbon solvents including high flash point solvents with a high aromatic content such as SOLVESSO 150 commercially available from Exxon (SOLVESSO is a trade mark) and AROMASOL H which consists essentially of a mixture of tri ethylbenzenes and is commercially available from Imperial Chemical Industries PLC (AROMASOL is a trade mark) . Especially preferred, however, on grounds of low toxicity and wide availability are hydrocarbon solvents of relatively low aromatic content such as kerosene, for example ESCAID 100 which is a petroleum distillate comprising 20% aromatics, 56.6% paraffins and 23.4% naphthenes commercially available from Exxon (ESCAID is a trade mark) . The conditions under which the solvent extraction is performed are chosen to suit the metal or metals present in the aqueous solution. It is generally desirable that conditions are selected such that any other metals present do not form stable complex compounds with the extractant compound in order that substantially only the desired metal is extracted from the aqueous solution. Since formation of the complex may involve the liberation of acid, it may be necessary to add, for example, alkali during the process to maintain the pH within a desired range but it is generally preferred to avoid this, especially in a continuously-operated process. It is a particular advantage of the process of the invention that zinc can be extracted selectively even in the presence of iron. Stages (3) and (4) of the process may conveniently be carried out by intimately contacting the solution of metal complex in the organic solvent obtained in stage (2) with an aqueous solution of a mineral acid at a suitable temperature, the two phases then being separated in conventional manner. The operations are usually performed at ambient temperature although somewhat higher temperatures, for example up to 100°C but preferably not more than 50°C, may be used if operationally convenient. The aqueous strip solution used in stage (3) preferably contains sulphuric acid, suitable strengths being from 100 to 250g of acid per dm 3 of solution. After removal of a convenient part of the metal by, for example, electrolysis, the recovered aqueous acid, containing residual metal salt, may be re-used in stage (3) of the process. The extractant compound regenerated in stage (3) may be recycled for use in stage (1) . Suitable relative volumes of organic to aqueous phases are those conventionally used in metal extraction processes and in the stripping stage will be typically not more than 10:1. The stripped organic layer, containing regenerated extractant compound and some residual metal, may be re-used in stage (1) of the process. The aqueous layer from stage (4) , containing metal salt, may be treated in any conventional manner to obtain the metal. The effectiveness of the compounds of the invention as extractants for zinc may be determined by the procedure of Test 1 which is described below. The extractant compounds of the invention are valuable in that, when subjected to Test 1, they are generally capable of providing an organic solution containing from 1500 to 4500 parts per million, often 3000 to 4500 parts per million and preferably 3600 to 4500 parts per million of zinc, a range which provides the most efficient combination of extraction and stripping. In the case of extractant compounds of the invention which provide a figure much above the 3600-4500 ppm range, it is advantageous to incorporate one of the above mentioned modifiers to have a weakening effect on the extractant. The invention is illustrated but not limited by the following Examples. Examples 1-5, relating to compounds in which no 2-alkylphenoxy group contains a tertiary alkyl substituent, are included for the sake of comparison. Example 1 This Example describes the preparation of 0,0' -bis (2- isopropyl-5-methylphenyl)chlorothiophosphate which is the chloro compound listed in Table 1 and the preparation of 0,0' -bis (2,4- dimethylphenyl) thiophosphoramide which is the amino compound listed in Table 1, and reaction of the chloro compound with the amino compound to give the compound of Formula 1 in which R X =R 2 = 2-isopropyl-5- methylphenoxy, and R 3 =R 4 = 2,4-dimethylphenoxy, which is the product of this Example. The General Method designated Test 1 of testing the product for its strength as an extractant for zinc, with the result which is ennumerated in Table 1, is also described. Sodium hydride (0.40M, 9.6g) was added in portions during 15 minutes to a stirred solution of 2-isopropyl-5-methylphenol (0.40M, 60g) in tetrahydrofuran (350 cm 3 ) in an atmosphere of nitrogen. During this addition the temperature rose to about 50°C. The solution was allowed to cool and was then added during 45 minutes under nitrogen atmosphere to a stirred solution of thiophosphoryl chloride (0.20M, 33.9g) in tetrahydrofuran (50 cm 3 ) which was maintained at - 40°C by external cooling. The reaction mixture was allowed to warm to ambient temperature when a sample analysed by HPLC indicated that reaction was complete. The chloro compound was isolated by diluting the mixture with diethyl ether (300cm 3 ) ,* the ether solution was extracted with water (three 100 cm 3 portions)and the organic layer was separated, dried with magnesium sulphate, filtered and concentrated by evaporation of the ether under reduced pressure yielding 0,0' -bis(2- isopropyl-5-methylphenyl) chlorothiophosphate which was an oil (73,lg) : 31 P NMR in CDC1 3 , singlet 56.7 ppm downfield of phosphoric acid. By the same procedure 2,4-dimethylphenol was reacted with sodium hydride and then with thiophosphoryl chloride to give a solution of 0,0' -bis (2,4-dimethylphenyl)chlorothiophosphate in tetrahydrofuran. This reaction intermediate was not isolated but, instead, ammonia gas was bubbled through the solution for 2 hours. Precipitated ammonium chloride was removed by filtration and the solution was concentrated by evaporation under reduced pressure yielding an oil which was 0,0' -bis (2,4- dimethylphenyl) thiophosphoramide: 31 P NMR in CDC1 3 , singlet, 59.6 ppm downfield of phosphoric acid. The final stage of the reaction was carried out as follows. The amino compound prepared as described above (0.05M, 16.05g, assuming 100% purity at M.W. 321) and the chloro compound also prepared as described above (0.05M, 19.8g assuming 100% purity at M.W. 396.5) were dissolved in a mixture of hexane (100 cm 3 ) and tetrahydrofuran (50 cm 3 ) and the solution was stirred whilst a suspension of sodium hydride (0.115M, 2.76g) in hexane (25 cm 3 ) was added during 30 minutes. The mixture was then stirred for 18 hours at ambient temperature (Note: in later Examples it was found that the reaction could be completed without detriment by boiling the mixture under reflux, at about 65-70°C, for 3 hours,* the necessary reaction time increased with increasing bulk of the substituents at position 2- of the phenoxy groups) . The reaction mixture was filtered and concentrated by evaporation of the solvents under reduced pressure yielding an oil which was the sodium salt of the crude reaction product. The crude reaction product was purified and isolated by the following general method. The oil was dissolved in hexane (100 cm 3 ) and the hexane solution was twice extracted with 100 cm 3 portions of a mixed solvent prepared by adding 5 parts by volume of water to 95 parts by volume of methanol. The hexane solution was then discarded. The methanolic solutions were combined and dilute sulphuric acid was added to reduce the pH to about 2.0. This solution was then extracted with hexane (two 100 cm 3 portions) and the hexane solutions were combined and dried (magnesium sulphate) and concentrated by evaporation of the hexane under reduced pressure yielding an oil (19.5g) which was the compound of Formula 1 in which R 1 =R 2 = 2- isopropyl-5-methylphenoxy and R 3 =R 4 = 2,4-dimethylphenoxy. The purity of this compound was estimated by potentiometric titration of a sample (0.3576g) dissolved in 50% aqueous tetrahydrofuran with 0.1 molar sodium hydroxide solution. The acidic (NH) proton was neutralised between pH 4.0 and pH 8.6, requiring 4.4 cm 3 .of alkali; hence it was calculated that the compound was 84% pure based on M.W. 681. The compound was examined for its strength in extraction of zinc by Test 1 described below with the result enumerated in Table 1 which indicates that although it is a good extractant for zinc, it does not have the very high strength ideally required. TEST 1 A 0.20 molar solution of the compound to be tested in ESCAID 100 is shaken with an equal volume of a 0.1 molar aqueous solution of zinc sulphate containing sufficient sulphuric acid to give an initial pH value of 2.0. Samples of the dispersion are withdrawn periodically and the aqueous layer is separated and analysed for zinc by titration with EDTA according to the usual procedure, until successive samples give the same zinc value denoting that equilibrium has been reached. The amount of zinc which has passed into the or-ganic solution, expressed as a concentration in parts of zinc by weight per million parts of solution by volume (ppm) is calculated. These results are listed in Table 1. If the compound to be tested was not sufficiently soluble in ESCAID then SOLVESSO 150 was used as solvent, and this is noted in the Table. EXAMPLES 2-5 The chloro compounds and amino compounds listed in Table 1 were prepared by the methods of Example 1, using as appropriate 2-sec- butylphenol, 2,4-dimethylphenol, 2-isopropyl-5-methylphenol or 2,6- dimethylphenol as starting materials. The chloro and amino compounds were then reacted together, also by the method of Example 1 to give the compounds of Formula 1 having the groups R 1 -R 4 listed. The strengths of these compounds as zinc extractants were examined by Test 1 with the results listed. The results show that the compounds of these Examples have strengths similar to the compound of Example 1. EXAMPLES 6-9 These Examples demonstrate compounds of Formula 1 in which two phenoxy groups each bearing a 2-tert-alkyl substituent are attached to the same phosphorus atom. The compounds, which are listed in Table l, were prepared by the methods of Example l using 2-tert- butyl-4-methylphenol or 2-tert-butylphenol as starting materials where required, with the following modifications: (i) in preparing the chloro compounds listed in Table 1 it was found advantageous to carry out the reaction without external cooling, and finally to boil the reaction mixture under reflux for 18 hours, (ii) in reacting the chloro compounds with the amino compounds it was found necessary to boil the reaction mixture under reflux for up to 72 hours to complete the reaction. The compounds of Formula 1 were assessed by Test 1 with the results listed in Table 1 which show that they are all stronger extractants for zinc than the products of Examples 1-5. EXAMPLE 10 This Example demonstrates the preparation of a compound of Formula 1 in which differently substituted phenoxy groups are attached to the same phosphorus atom. Preparation of the chloro compound for this reaction was carried out as described in Example 9. Sodium hydride (0.60M, 14.4g) was added during 10 minutes to a stirred solution of 2-tert-butvl-5-methvlohenol (0.60M, 98.4g) in hexane (400 cm 3 ) and tetrahydrofuran (50 cm 3 ) whilst the temperature was maintained below 30°C by external cooling. This solution was then added during 45 minutes to a stirred solution of thiophosphoryl chloride (0.60M, 101.7g) in hexane (100 cm 3 ) under nitrogen atmosphere whilst the reaction temperature was maintained at -40°C. The reaction mixture was allowed to warm to ambient temperature to complete the reaction and after 30 minutes was cooled again to -40°C. To this solution was then added a solution of sodium 4-tert-nonvlphenoxide prepared by adding sodium hydride (0.60M, 14.4g) to commercial mixed- isomer 4-tert-nonvlphenol (0.60M, 132g) dissolved in hexane (400 cm 3 ). The addition lasted 30 minutes, and the temperature during addition was maintained at -40°C. The reaction mixture was allowed to warm to ambient temperature when analysis by HPLC indicated that reaction was complete. The solution was extracted with water (three 200cm 3 portions) , dried with magnesium sulphate, filtered and concentrated by evaporation of the hexane under reduced pressure, yielding an oil which was substantially 0- (2-tert-butyl-5-methylphenyl) -O' - (4-tert- nonylphenyl)chlorothiophosphate ( 31 P NMR in CDC1 3 , multiplet 55.3 ppm down field of phosphoric acid) . This material (130g) was dissolved in tetrahydrofuran (400 cm 3 ) and ammonia gas was bubbled through the solution for 45 minutes whilst the temperature was kept below 35°C by external cooling. Analysis by HPLC then showed complete conversion of the chloro compound. The mixture was diluted with diethyl ether (400 cm 3 ) and extracted with water (three 200 cm 3 portions) , dried with magnesium sulphate and concentrated by evaporation of the solvents under reduced pressure yielding the amino compound listed in Table 1 (115.3g); 31 P NMR in CDC1 3 multiplet, 59.8 ppm downfield of phosphoric acid. The amino compound (0.22M, 101.4g) was reacted with 0,0'- bis (2-tert-butvl-5-methvlphenoxv) chlorothiophosphate (0.22M, 93.4g) and sodium hydride (0.55M, 13.2g) in tetrahydrofuran (200 cm 3 ) solution; it was found necessary to boil this reaction mixture under reflux for 70 hours before HPLC analysis showed that the starting materials had been almost completely converted (see Example 9, (ii) ) . After purification and isolation as described in Example 1 the reaction product was obtained (103.1g), but found by titration to have a purity of only 57% (M.W. 833) . Accordingly it was dissolved in toluene (400 cm 3 ) and the toluene solution was extracted twice with 2M sodium carbonate solution (200 cm 3 portions) . The toluene solution was then extracted twice with aqueous methanol (95ppv methanol mixed with 5 ppv water, 200 cm 3 portions) and the methanolic solutions were combined and extracted with toluene (three 100 cm 3 portions) . Concentrated hydrochloric acid (40 cm 3 ) was then added to the methanolic solution and this acidified solution was extracted with hexane (400 cm 3 ) . The hexane solution was extracted three times with fresh aqueous methanol solution (100 cm 3 portions) . The hexane solution was then dried with magnesium sulphate, filtered and concentrated by evaporation of solvents yielding the product of this Example (49.6g) which was found by titration to be 87% pure. The performance of this compound in Test 1 (Table 1) showed it to be a very strong extractant for zinc. EXAMPLES 11-13 These compounds of Formula 1 were prepared by the methods of Example 10, except that in the final reaction stage the second purification step was omitted as unnecessary. The compounds together with their performance in Test 1 are set out in Table l. The results show that the products of Examples 6-10 and Example 13, in which two phenoxy groups having 2-tert-alkvl substituents are joined to the same phosphorus atom are stronger extractants for zinc than the products of Examples 11 and 12. EXAMPLES 14-24 The products of these Examples, which are set out in Table 1, each contain a substituted phenoxy group and a phenyl group attached to the same phosphorus atom. They were prepared by reacting the chloro compounds set out in Table 1 with the corresponding amino compounds set out in Table 1 in the presence of sodium hydride. The chloro compounds required for these reactions are prepared by the methods set out in Examples 1-13. The amino compounds required for these reactions are prepared by reacting phenylphosphonothioic dichloride (Ph.PS.Cl 2 , obtainable from Jansen Chemical Company) firstly with the appropriate substituted phenol and secondly with ammonia by procedures detailed for Example 24 which follow. Sodium hydride (0.60M, 14.4g) was added to a solution of 2-tert-t-butvlphenol (0.6M, 90.Og) in tetrahydrofuran (250 cm 3 ) during 10 min at -40°C. This solution was then added during 30 min. to a solution of thiophosphoryl chloride (0.60M, 101.7g) in hexane (250 cm 3 ) keeping the temperature throughout at -40°C. The solution was stirred at this temperature for a further 30 min. A solution of the sodium salt of 2-sec-butvlphenol was then prepared from 90.Og of the phenol, sodium hydride (14.4g) and tetrahydrofuran (250 cm 3 ) and added to the reaction mixture, again maintaining the temperature at -40°C. The mixture was allowed to warm to room temperature. Hexane (250 cm 3 ) was added and the mixture was extracted with water (three 200cm 3 portions) . The hexane solution was dried with magnesium sulphate and concentrated by evaporation of hexane under reduced pressure yielding an oil (231g) which is the chloro compound of Example 24. By 31 P NMR the product prepared in this way contained < 9% of the two isomers having identical phenoxy groups. Sodium hydride (0.6M, 14.4g) was added to a stirred solution of 2-tert-butvlphenol (0.60M, 90.Og) in tetrahydrofuran (250 cm 3 ) , in portions during 10 minutes so as to keep the reaction temperature below -40°C. This solution was then added during about 40 min. to a solution of phenylphosphonothioic dichloride (0.60M, 126.6g) in hexane (250 cm 3 ) held at -40°C. The mixture was then allowed to warm to ambient room temperature during about 1 hr. Analysis by HPLC indicated that reaction was complete. Ammonia was bubbled through the mixture for about 45 min. (raising the temperature from ambient to 40°C) when HPLC showed conversion of all the chloro compound to amino compound. The mixture was extracted with water (three 200cm 3 portions) , and the organic solution was dried with magnesium sulphate and filtered and the hexane was evaporated under reduced pressure leaving an oil (178g) which crystallised on standing, which is the amino compound of Example 24. The chloro compound described above (198.3g), and the amino compound also described above (152.5g) were dissolved in hexane (250 cm 3 ) and tetrahydrofuran (250 cm 3 ) . Sodium hydride (30.Og) was added during 15 min. at room temperature. After addition was complete the mixture was stirred and boiled under reflux at 60-70°C for 18 hrs. The mixture was cooled to room temperature and isopropanol (80 cm 3 ) was added cautiously (frothing occurred) to destroy excess sodium hydride. The product was purified and isolated as follows: the reaction mixture was extracted twice with 300 cm 3 portions of a solution comprising methanol (540 cm 3 ) and water (60 cm 3 ) . The methanolic solutions were combined, and extracted with hexane (200 cm 3 ) , and the hexane extract was discarded. The methanol solution was acidified with concentrated hydrochloric acid (100cm 3 ) and water (100 cm 3 ) was added. The product was extracted into hexane (400 cm 3 ) and the hexane was washed with three 100 cm 3 portions of methanol/water made up as previously described. The hexane solution was dried with magnesium sulphate and the hexane was evaporated under reduced pressure leaving an oil (213g) which is the product of Example 24. The purity was estimated by titration as 89.1% of theoretical for M.W. 665. 31 P NMR in CDC1 3 : a triplet of doublets centred at 45.1ppm (phosphorus 1) and a doublet of multiplets centred at 65.0 ppm (phosphorus 2) ,* measurements are downfield of phosphoric acid. The performance of the products of these Examples in Test 1 which is set out in Table 1 shows them all to be very strong extractants for zinc. The product of Example 24 was found to have particularly high solubility in hydrocarbon solvents. EXAMPLE ' 25 This Example describes the preparation of a compound of Formula 1 in which one phenyl group is attached to each phosphorus atom. The chloro compound listed in Table 1 was prepared by reacting 2-tert-butyl-4-methylphenol (1.0M, 164.3g) with sodium hydride (1.0 M, 24.Og) and then with phenylphosphonothioic dichloride (1.0.M, 211g) as described in Example 24. On this occasion, the reaction solution was not treated with ammonia but, instead, was concentrated by evaporation of the tetrahydrofuran under reduced pressure, and the concentrate was redissolved in ethyl acetate (400 cm 3 ) and the solution was extracted with water (three 100 cm 3 portions) . The ethyl acetate solution was dried with magnesium sulphate, filtered , and again concentrated yielding the chloro compound which was a crystalline solid (281g, m.p. 71-73°C) . Part of this chloro compound (169.3g) was reacted with ammonia as described in Example 1 to give the amino compound listed in Table 1 (130g, m.p. 108.5-lll°C) . This amino compound (0.2M, 63.8g) was then reacted with a further part of the chloro compound (0.2M, 67.7g) and sodium hydride (0.5M, 12.Og) in tetrahydrofuran (400 cm 3 ) solution. It was necessary to heat the solution at 50-60°C for two hours to complete the reaction. The product was purified and isolated by the procedure of Example 1 but using an aqueous methanol solution of altered composition, i.e. 90 parts methanol to 10 parts of water, yielding the compound of Formula 1 in which R X =R 3 = 2-tert-butvl-4- methylphenoxy and R 2 =R 4 = phenyl (60.6g, purity 80% of theoretical for M.W. 621) . The behaviour of this compound in Test 1, which is enumerated in Table 1, shows it to be a stronger extractant than the product of Examples 1-5. Example 26 Firstly this Example describes the preparation of 2- methyl-4-tert-nonylphenol. Secondly this Example describes the preparation of diphenylphosphonothioic amide (Ph 2 PS.NH 2 ) and its use in preparation of a compound of Formula 1 in which two phenyl groups are attached to the same phosphorus atom. Propylene trimer (2.0M, 252g) and 2-methylphenol (2.0M, 216g) and an activated Fullers earth catalyst (FULLCAT 22B supplied by Laporte Industries, 5.4g) and phosphoric acid (4 drops) were stirred and heated at 80°C for 48 hours. The mixture was then allowed to cool, filtered and distilled, yielding 2-methyl-4-tert-nonvlphenol (245g) as the fraction of b.p. 114-132°C under a pressure of 0.2-0.3mm of mercury. Thiophosphoryl chloride (0.45M, 76.9g) was added to chlorodiphenylphosphine (0.45M, lOOg) in nitrogen atmosphere during ten minutes so that the reaction temperature did not rise above 70°C. The solution was allowed to stand for 18 hours and then distilled yielding diphenylphosphonothioic chloride (100.6g) as the fraction of b.p. 178-180°C at a pressure of 0.6mm of mercury. All this compound was reacted with ammonia by the procedure of Example 1 yielding diphenylphosphonothioic amide ( 84,3g) which is a white crystalline solid*. 31 P NMR in CDC1 3 , singlet 53.8ppm downfield of phosphoric acid. 2-Methvl-4-tert-nonylphenol was reacted with thiophosphoryl chloride by the procedure of Example 5, except that hexane was used as the solvent instead of tetrahydrofuran, to give the chloro compound listed in Table 1. This chloro compound (0.1M, 56.5g) was reacted with diphenylphosphonothioic amide (0.1M, 23.3g) and sodium hydride (0.2M, 8.0g) using the procedure of Example 1 to give the compound of Formula 1 in which R 1 =R 2 = 2-methyl-4-tert-nonylphenoxy and R 3 =R 4 = phenyl (66.lg, purity 88% of theoretical for M.W. 761). The result of Test 1 shows this compound to be a stronger extractant for zinc than the products of Example 25, teaching that stronger extractants are obtained when two phenyl groups are attached to the same phosphorus atom than when each phenyl group is attached to a different phosphorus atom. Example 27 The procedure of Example 26 was used to prepare 2-sec- butyl-4-tert-nonylphenol (b.p. 148-150°C at a pressure of 1.5mm of mercury) from 2-sec-butvlphenol and propylene trimer. This compound was further reacted with phenylphosphonothoic dichloride using the procedure of Example 24 to give the chloro compound listed in Table 1 ( 31 P NMR in CDC1 3 , multiplet 82.6ppm downfield of phosphoric acid) . This chloro compound was then reacted with diphenylphosphonothioic amide using the procedure of Example 26 to give the compound of Formula 1 in which R * -= 2-sec-butyl-4-tert-nonylphenoxy and R 2 =R 3 =R 4 = phenyl. This compound was subjected to Test 1 with the result listed in Table 1 which shows it to be a strong extractant for zinc. TABLE It Preparation of the Compounds of Formula I in which R x -R 4 are the Groups listed, and the Strengths of these Compounds as Zinc Extractants EXAMPLE 28 In order to exemplify further the utility of materials of this invention for the extraction of metals, a distribution curve was obtained for the extraction of zinc from an aqueous metal bearing solution by an organic solution of the extractant. This was done by equilibrating various volume ratios of extractant and metal bearing feed solutions, separating and analysing for the metal contained in each phase. An organic solution was prepared containing 0.5 moles per litre of the extractant of Example 24 in the hydrocarbon solvent Escaid 100. A simulated high concentration zinc feed solution was prepared that contained 20.1 g/1 zinc, 12.5 g/1 ferric iron, 0.47 g/1 calcium, and 2.6 g/1 magnesium in an aqueous sulphate medium at pH 1.8. In a series of experiments, various volume ratios of the organic extractant solution and aqueous feed solution were equilibrated by vigorous stirring at 25°C for a period of 24 hours. The phases were then allowed to disengage, separated, filtered and analysed for zinc. The distribution of zinc between the organic and aqueous phases after contacting at various volume ratios was found to be as follows: TABLE 2 These data demonstrate the ability of the reagent to attain both high loadings of zinc in the organic phase and high recovery of zinc from the aqueous phase. In practice this would be achieved by a number of equilibration stages with the organic and aqueous flows running counter current wise. In a solvent extraction process for the recovery of a metal, it is essential that the extractant is not only capable of extracting metal efficiently from the aqueous feed solution, but that the metal can be recovered subsequently from the metal loaded organic phase by a stripping operation. Ideally, for use of the extractant in a process for zinc recovery based on solvent extraction combined with electrowinning, it is desirable that the stripping is carried out with an acidic aqueous solution such as a spent electrowinning electrolyte. In order to demonstrate this, a portion of the extractant solution of the composition described in the first part of this Example was loaded with zinc by contacting for at least 12 hours one part by volume with four parts by volume of the aqueous feed solution also described in the first part of this Example. Portions of this zinc loaded organic solution were then contacted at various volume ratios with an aqueous strip solution containing 30g/l zinc and 180 g/1 sulphuric acid. Contacting was carried out by vigorous stirring at 50°C for 2 hours. The phases were then separated and each analysed for zinc. The distribution of zinc after stripping at various volume ratios was found to be as follows: TABLE 3 EXAMPLE 29 In order to exemplify further the ability of compounds of the invention to be able to extract zinc with high selectivity over iron and for the zinc to be stripped subsequently with acidic zinc electrolyte solution, a number of compounds were loaded with zinc, and then stripped as described below. One part of a 0.5 molar solution of extractant in Escaid 100 was contacted by vigorous stirring for 24 hours at 25°C with four parts of the aqueous zinc feed solution of composition given in Example 28. The phases were allowed to disengage, separated, the organic filtered and analysed for both zinc and iron. A portion of the zinc loaded organic phase was then contacted with an aqueous strip solution as used in Example 28 in the ratio of 15 parts of organic to 7 parts of strip solution. Contacting was carried out by vigorous stirring for 2 hours at 50°C. After the phases had disengaged, the phases were separated, filtered, and analysed for zinc. The results for the compounds tested in this way are as follows: TABLE 4 These results clearly show the ability of solutions of the extractants to extract zinc, with high selectivity over iron, and to transfer it satisfactorily to an acidic strip solution. EXAMPLE 30 Solutions were prepared containing (A) 0.5 moles per litre of the extractant of Example 24 in the hydrocarbon solvent Escaid 100. In addition, one of the solutions (B) contained 50 g/litre (0.25 molar) of isotridecyl alcohol added as modifier. Yet a third solution of the extractant (C) contained as modifier 72 g/1, being 0.25 moles per litre, of the ester 2,2,4-trimethyl-l,3-pentanediol diisobutyrate available commercially under the name KODAFLEX TXIB (Eastman Kodak) . Portions of each of these extractant solutions were contacted by vigorous stirring for 24 hours at 25°C with the aqueous zinc feed solution described in Example 28 and in the ratio of 10 parts of extractant solution contacted with 20 parts of aqueous feed. The phases were allowed to disengage, separated and portions set aside for analysis. Portions of each organic phase, pre-loaded with zinc feed solution were then stripped by contact at an organic to aqueous phase ratio of 10 parts organic to 20 parts aqueous using the conditions described in Example 28. After 2 hours, the phases were allowed to disengage, separated and analysed. The results, in terms of zinc in the organic phase after extraction and again, after stripping, are as shown in the table below. TABLE 5 Modifier Zinc in loaded Zinc in stripped Zinc extractant (g/1) extractant (α/1) transferred (g/1) A) None 13.06 5.63 7.43 B) isotridecylalcohol 12.39 3.94 8.45 C) Kodaflex TXIB (ester) 11.98 3.18 8.80 These results show that under the same strip conditions for each extractant composition, more zinc is removed at the strip stage in the presence of the modifier than when the modifier is absent. There is also some reduction in the amount of zinc loaded at the extraction stage, but this is outweighed by the improvement at stripping, so that there is a net gain in the amount of zinc transferred between the extraction and stripping stages. EXAMPLE 31 This Example demonstrates the preparation of a compound of Formula I in which two optionally substituted 2-tert-alkylphenoxy groups are attached to the same phosphorus atom and two optionally substituted alkyl groups are attached to the other phosphorus atom. The Example describes the preparation of bis- (2- pentyl)chlorothiophosphate, which is the chloro compound listed in Table 6, and the reaction of the chloro compound with 0,0'-(bis-2- tert-butvlphenoxy)thiophosphoramide, which is the amino compound listed in Table 6 : A solution of 2-pentylmagnesium bromide was prepared by adding 2-bromopentane (2554.74g, 16.93M) to magnesium turnings (422g, 17.6M) in diethyl ether over a period of 2 hours. The temperature of the reaction mixture was maintained at 38-40°C by external cooling. The diethyl ether solution of 2-pentylmagnesium bromide was then added to a solution of phosphorus trichloride (946g, 6.88M) in diethyl ether (946 cm 3 ) during 3 hours. Throughout the addition, the temperature of the reaction mixture was maintained at -20°C or lower by external cooling. After the addition was complete, the reaction mixture was stirred at 0°C for 2 hours, over which time, a greyish solid resulted. Water (3000cm 3 ) was added to the reaction mixture over a period of 2 hours and the temperature was allowed to rise freely to 20°C. The diethyl ether solution was separated and washed with brine (1 x 2000cm 3 ) , then with brine/Na 2 Co 3 (15% wt Vol NaCl, 2% wt.vol. Na 2 C0 3 ; 5 x 3000cm 3 ) and again with brine (1 x 2000cm 3 ) . The diethyl ether solution was then concentrated by evaporation under reduced pressure. Low boiling impurities were removed by heating the residue under reduced pressure (25 mmHg) at a temperature of 80°C for 30 minutes. The clear oil (541.2g) was taken as the product bis- (2- pentyl)chlorophosphine 31 P, n.m.r in CDC1 3 , singlet at 130.7 ppm. Thiophosphoryl chloride (90.Og) was added dropwise to the bis- (2- pentyl) chlorophosphine, under a nitrogen atmosphere, over a period of 90 minutes. The temperature of the mixture rose to 45°C over this time. The reaction mixture was then cooled to ambient temperature and stirred for a further 2 hours. The PC1 3 by-product formed was distilled from the reaction mixture under reduced pressure. The remaining clear oil was taken as the product, bis (2- pentyl)chlorothiophosphate, 31 P n.m.r in CDC1 3 , singlet at 120.6 ppm. The amino compound, 0,0' -bis (2-tert- butylphenoxy)thiophosphoramide was prepared from bis (2-tert- butylphenoxy)chlorothiophosphate by the method of Example 1, and 0,0'- bis (2-ter -butylphenoxy)chlorothiophosphate was also prepare by the method of Example 1. The compound of Formula 1 was prepared by reacting bis (2- pentyl) chlorothiophosphate and 0,0' -bis (2-tert- butylphenoxy)thiophosphoramide by the method of Example 1. The compound isolated was a brown oil (44.04g) . The purity of the compound was estimated by titration as 89% of theoretical for M.W. of 580.6. 31 P n.m.r. in CDC1 3 , having two sets of multiplets centred at 41-.80 ppm and 90.85 ppm. The performance of the product of this Example in Test 1, which is set out on Table 6, shows it to be a very strong extractant for zinc. Example 32-33 These compounds of Formula I were prepared by the methods of Example 31. The compounds together with their performance in Test 1 are set out in Table 6. The results show that the compounds of Examples 32 & 33 are weaker extractants for zinc than the product of Example 31. Example 34 This Example describes the preparation of compounds of Formula 1, wherein R 3 and R 4 are alkoxy groups -and R 1 and R 2 are 2- alkylphenoxy groups. The Example describes the preparation of 0,0'- (bis-2-ethylhexyl)chlorothiophosphate, its conversion to phosphoramide and the preparation of 0,0' - (bis-2-tert-butylphenoxy)thiophosphoryl chloride. The final stage is the reaction of chloro and amino compound to produce the compound of Formula I where R 1 = R 2 = 2- ethylhexyloxy and R 3 =R 4 = 2-tert-butylphenoxy. To 0,0' - (bis-2-ethylhexyl)phosphorodithioate (0.7M, 254g) , sulphuryl chloride (1.1M, 150g) was added during 2 hours. The temperature rose to 40°C over this time. The reaction mixture was then stirred at ambient temperature for a further 10 hours, after which time a sample analysed by GC indicated the reaction was complete. The excess sulphuryl chloride was removed by vacuum evaporation (0.2 mmHg) of the crude product, at ambient temperature, for 2 hours. The oil residue was the product 0,0' - (bis-2-tert- ethylhexyl)chlorothiophosphate (225.24g), 31 P n.m.r. in CDC1 3 singlet at 67.28 ppm. The chloro compound (lOOg) was dissolved in tetrahydrofuran (250 cm 3 ) and ammonia gas was bubbled through the solution over 2 hours. Analysis of a sample by GC showed complete conversion of the chloro compound. Precipitated ammonium chloride was filtered from the mixture and the tetrahydrofuran solution was evaporated under reduced pressure to leave 0,0'-(bis-2- ethylhexyl)phosphoramide (85.45g) as an oil. 31 P n.m.r. in CDC1 3 singlet at 70.64 ppm. The amino compound (0.2M, 67.4g) was reacted with 0,0'- (bis-2-tert-butylphenoxy) chlorothiophosphate, prepared as described in Example 10, (0.2M, 79.3g) and sodium hydride (0.46M, 11.06g) in tetrahydrofuran (500cm 3 ) and refluxed for 12 hrs. After purification and isolation as described in Example 1, the reaction product was obtained (90.3g) , which was found by titration to be 89% pure. The purity of the sample was further improved to 93% by redissolving the product in hexane and washing the solution with a methanol/H 2 0 (90% Vol : 10% Vol) mixture 4-5 times. The purified product was isolated by evaporation under reduced pressure. 31 P n.m.r. in CDC1 3 : a doublet of doublets centred at 41.75ppm & 59.23ppm. The performance of the product in Test 1, which is listed in Table 6, showed it to be a very strong extractant for zinc. Examples 35-38 The compounds of Formula I, listed in Table 6, were prepared by the methods of Example 34 using, where appropriate, 0,0'- (bis-1,3-dimethylbutoxy)phosphorodithioate, 0,0' - (bis- ethoxy)phosphorodithioate, 0,0' - (bis-n-propoxy)phosphorodithioate, 0,0' - (bis-isopropoxy)phosphorodithioate and 0,0' - (bis-2-tert- butylphenoxy)chlorothioate as the starting materials. The chloro and amino compounds were then reacted together, by the methods of Example 1 to give compounds of Formula 1. The compounds, together with their performance in Test 1, are set out in Table 6. The results show that the compounds of these Examples have strengths similar to the compound of Example 34. Example 39 This Example demonstrates the preparation of a compound of Formula I in which 2-tert-butylphenoxy and isopropoxy groups are attached to the same phosphorus atom. Preparation of a chloro compound for this reaction was carried out as described as in Example 10. A suspension of sodium hydride (1.0M, 24g) in tetrahydrofuran (150cm 3 ) was added portionwise to a stirred solution of 2-ter -butvlphenol (150g) in tetrahydrofuran (250cm 3 ) over a period of 2 hours, whilst the temperature was maintained below 30°C by external cooling. This solution was then added during 2 hours to a stirred solution of thiophosphoryl chloride (1.0M, 169.4g) in tetrahydrofuran (350cm 3 ) whilst the reaction temperature was maintained at -50°C. The reaction mixture was allowed to warm up to ambient temperature to complete the reaction, and after 30 minutes, was cooled to -30°C. To this solution was then added a solution of sodium isopropoxide prepared by adding a suspension of sodium hydride (1.0M, 24g) in tetrahydrofuran (150cm 3 ) to a solution of commercial isopropanol (60g) in tetrahydrofuran (250cm 3 ) . The addition lasted 2 hours and the temperature was maintained below -30°C. The reaction mixture was allowed to warm to ambient temperature after which analysis by GC indicated that reaction was complete. The tetrahydrofuran was removed by vacuum evaporation and the residue was diluted with diethyl ether (1000cm 3 ) . The diethyl ether solution was washed with water (2 x 500cm 3 portions) , dried with magnesium sulphate, filtered and concentrated by evaporation of the diethyl ether under reduced pressure. The remaining oil was substantially O- (2-tert-butvlphenvl) -O' - (2-isopropyl) chlorothiophosphate. 31 P n.m.r in CDC1 3 singlet at 57.63ppm. This compound (52.42g) was dissolved in tetrahydrofuran (350cm 3 ) and ammonia gas was bubbled through the solution for 2 hours. Analysis of a sample by GC showed complete conversion of the chloro compound. Precipitated ammonium, chloride was removed by filtration, and the tetrahydrofuran solution was evaporated under reduced pressure to give a brown oil (42.00g) which was the expected amide. The amide (0.1M, 28.7g) was reacted with the chloro compound (0.1M, 30.65g) and sodium hydride (0.23M, 5.52g) in tetrahydrofuran (150cm 3 ) . The reaction mixture was heated under reflux for 12 hours until analysis by HPLC indicated that the reaction was complete. The product was purified and isolated as described in Example 1. The reaction product obtained (30.7g) was found by titration to have a purity of 70% for a M.W. of 557. The product was further purified by dissolving it in hexane (250cm 3 ) and washing the solution with a mixture of 90% vol MeOH and 10% vol H 2 0 (4-5 x 100cm 3 portions) . The hexane solution was dried over magnesium sulphate, filtered and the hexane was removed by evaporation under reduced pressure to give a brown oil (15.25g), which was found by titration to be 89.7% pure : 31 P n.m.r in CDC1 3 , a singlet (with fine splitting) at 50.05 ppm. The performance of this compound in Test 1, which is enumerated in Table 6, shows it to be as strong as the products of Examples 1-5. Example 40 The chloro compound listed in Table 6 was prepared by the method of Example 39. The amino compound listed in Table 6 was prepared by the method of Example 34. The chloro and amino compounds were reacted together by the methods of Example l to give the compound of Formula I having the groups R 1 - R 4 listed. The purity of the compound was estimated by titration as 86% for MW of 557. Example 41 This Example demonstrates the preparation of a compound of Formula I, wherein R 1 is a 2-alkylphenoxy group, R 2 is a phenyl group and R 3 and R 4 are alkoxy groups. The chloro compound listed in Table 6 was prepared by the methods of Example 24. The amino compound listed in Table 6 was prepared by the methods of Example 34. The chloro and amino compounds were reacted together by the methods of Example 1 to give the compound of Formula I listed in Table 6. The purity of the compound was estimated by titration as 91%. The performance of this compound in Test 1, which is enumerated in Table 6 shows it to be a weak extractant for zinc. Example 42 This Example demonstrated the preparation of a compound of Formula I by the methods described in Example 1. The chloro compound listed in Table 6 was prepared by the methods of Example 31. The amino compound was prepared by the methods of Example 24. The chloro and amino compounds were reacted together by the methods of Example 1. The purity of the compound having R 1 - R 4 listed in Table 6 was 84% for a compound of MW 509. The behaviour of this compound in Test 1, is enumerated in Table 6. Example 43 This Example demonstrates the preparation of a compound of Formula I by the methods described in Example 1. The chloro compound listed in Table 6 was prepared by the methods of Example 39. The amino compound listed in Table 6 was prepared by the methods of Example 10. The chloro and amino compounds were reacted together by the methods of Example 1. The purity of the compound having R 1 - R 4 listed in Table 6 was 76% for a compound of MW 647. The behaviour of this compound in Test 1, is enumerated in Table 6. TABLE 6 Example 44 An aqueous solution containing up to 350 parts per million of each of 15 different metals or metalloids and sufficient nitric acid to give a pH value of 2.0 was made up. The metals were taken as their nitrate or acetate salts except for arsenic which was taken as the trioxide. This solution was stirred rapidly with a 0.1 molar solution of the product of Example 24 in ESCAID 100, for one hour at 20-25°C. The aqueous and organic phases were separated and each was analysed for metals content with the results tabulated below : Metal Concentration of metal found, in parts per million In the aqueous phase In the or anic phase The results show that at an initial pH value as low as 2.0, zinc, bismuth, cadmium, silver and mercury are strongly extracted and may be separated from the other metals listed. Example 45 In a separate test, a 0.1 molar solution of the product of Example 26 in ESCAID 100 was stirred for one hour at 20-25°C with the mixed metal solution of Example 44. the aqueous and organic phases were separated and the organic phase was analysed for metal content with the results tabulated below : Concentration of metal found in parts per million l In the organic phase

Claims: CLAIMS 1 . A compound of the formula : wherein R 1 is an optionally substituted 2-alkylphenoxy group, each of R 2 , R 3 and R 4 is a group selected from optionally substituted 2- alkylphenoxy, optionally substituted phenyl, optionally substituted alkyl and optionally substituted alkoxy and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R 1 and R 2 is 2-isopropyl-4-tert- nonylphenoxy and each of R 3 and R 4 is phenyl and the compound wherein R 1 is 2-methyl-4-tert-nonylphenoxy, R 2 is 2,4-dimethylphenoxy and each of R 3 and R 4 is phenyl. 2. A compound according to claim 1 wherein at least one 2- alkylphenoxy groups is a 2-tert-alkylphenoxy group. 3. A compound according to claim 2 wherein at least two 2- alkylphenoxy groups are 2-tert-alkylphenoxy groups. 4. A compound according to any one of claims 1 to 3 wherein R 1 is an optionally substituted 2-alkylphenoxy group, each of R 2 , R 3 and R 4 is either an optionally substituted 2-alkylphenoxy group or an optionally substituted phenyl group and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R 1 and R 2 is 2-isopropyl-4-tert- nonylphenoxy and each of R 3 and R 4 is phenyl and the compound wherein R 1 is 2-methyl-4-1ert-nonylphenoxy, R 2 is 2,4-dimethylphenoxy and each of R 3 and R 4 is phenyl. 5. A compound according to claim 4 wherein at least one of R 2 , R 3 and R 4 is an optionally substituted 2-alkylphenoxy group. 6. A compound according to claim 5 wherein at least two optionally substituted 2-alkylphenoxy groups have a tertiary alkyl substituent. 7. A compound according to claim 5 or claim 6 wherein each of R 1 and R 2 is an optionally substituted 2-alkylphenoxy group and each of R 3 and R 4 is an optionally substituted phenyl group. 8. A compound according to claim 5 wherein each of R 1 and R 3 is an optionally substituted 2-tert-alkylphenoxy group and each of R 2 and R 4 is an optionally substituted phenyl group. 9. A compound according to claim 5 wherein each of R 1 , R 2 and R 3 is an optionally substituted 2-alkylphenoxy group and R 4 is an optionally substituted phenyl group. 10. A compound according to claim 9 wherein one of R 1 , R 2 and R 3 is a 2-1ert-alkylphenoxy group and each of the other two, independently, is a 2-tert-alkylphenoxy group or a 2-sec-alkylphenoxy grou . 11. A compound according to claim 5 wherein each of R 1 , R 2 , R 3 and R 4 is an optionally substituted 2-alkylphenoxy group. 12. A compound according to claim 11 wherein two or three of R 1 , R 2 , R 3 and R 4 are 2-tert-alkylphenoxy groups. 13. A compound according to claim 12 wherein only R 1 and R 2 are 2-tert-alkylphenoxy groups. 14. A compound according to claim 13 wherein each of R 3 and R 4 is a 2-sec-alkylphenoxy group. 15. A compound according to any one of claims 1 to 3 wherein R 1 is an optionally substituted 2-alkylphenoxy group, at least one of R 2 , R 3 and R 4 is an optionally substituted alkyl or optionally substituted alkoxy group, any remaining group or groups from R 2 , R 3 and R 4 being selected from optionally substituted 2-alkylphenoxy and optionally substituted phenyl and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent. 16. A compound according to claim 15 wherein R 1 is an optionally substituted 2-tert-alkylphenoxy group, R 2 is an optionally substituted alkoxy group and each of R 3 and R 4 , independently, is an optionally substituted 2-alkylphenoxy group or an optionally substituted alkoxy group. 17. A compound according to claim 16 wherein at least one of R 3 and R 4 is a 2-tert-alkyphenoxv group. 18. A compound according to claim 15 wherein R 1 is an optionally substituted 2-tert-alkylphenoxy group, R 2 is an optionally substituted alkoxy group or an optionally substituted phenyl group and R 3 and R 4 are optionally substituted alkoxy groups. 19. A compound according to claim 15 wherein R 1 is an optionally substituted 2-tert-alkvlphenoxv group, R 2 is an optionally substituted 2-alkylphenoxy group or an optionally substituted phenyl group and R 3 and R 4 are optionally substituted alkyl groups. 20. A process for extracting metal values from aqueous solutions of metal salts which comprises contacting the aqueous solution with an organic phase comprising a compound as defined in any one of claims 1 to 3. 21. A process for extracting metal values from aqueous solutions of metal salts which comprises contacting the aqueous solution with an organic phase comprising a compound as defined in any one of claims 4 to 14. 22. A process for extracting metal values from aqueous solution of metal salts which comprises contacting the aqueous solution with an organic phase comprising a compound as defined in any one of claims 15 to 19. 23. A process according to any one of claims 20 to 21 which comprises a sequence of stages comprising : (1) contacting the aqueous solution containing metal values with a solution of an extractant compound of Formula I in a water-immiscible organic solvent whereby to extract metal values into the solvent in the form of a complex of the metal with the extractant,* (2) separating the solvent phase containing metal, complex from the extracted aqueous phase; (3) contacting the solvent phase containing metal complex with an aqueous strip solution whereby the metal complex is unstable and metal ions transfer into the aqueous phase, and (4) separating the aqueous phase containing metal ions from the stripped solvent phase. 24. A process according to any one of claims 20 to 23 wherein the metal is zinc.

Inventors: CAMPBELL JOHN, CUPERTINO DOMENICO CARLO, DALTON RAYMOND FREDERICK, QUAN PETER MICHAEL, CAMPBELL, JOHN, CUPERTINO, DOMENICO, CARLO, DALTON, RAYMOND, FREDERICK, QUAN, PETER, MICHAEL
Applicants: CAMPBELL JOHN, CUPERTINO DOMENICO CARLO, DALTON RAYMOND FREDERICK, QUAN PETER MICHAEL, ZENECA LTD, CAMPBELL, JOHN, CUPERTINO, DOMENICO, CARLO, DALTON, RAYMOND, FREDERICK, QUAN, PETER, MICHAEL, ZENECA LIMITED
Application year: 1994
Main classification: C07F9
IPC codes: C07F9, C22B3, C22B19